The present invention generally relates to photolithography process development, and more particularly relates to a system and method for selecting one of various available OPC designs for a feature pattern based upon evaluation of measurement data obtained using, for example, atomic force microscopy.
The minimum feature sizes of integrated circuits are continuously decreasing in order to increase the packing density of the various semiconductor devices formed thereby. With this size reduction, however, various steps within the integrated circuit fabrication process become more difficult. One such area within the semiconductor fabricating process which experiences unique challenges as feature sizes shrink is photolithography.
Photolithography involves selectively exposing regions of a resist-coated silicon wafer to form a radiation pattern thereon. Once exposure is complete, the exposed resist is developed in order to selectively expose and protect the various regions on the silicon wafer defined by the exposure pattern (e.g., silicon regions in the substrate, polysilicon on the substrate, or insulating layers such as silicon dioxide).
An integral component of a photolithography or pattern transfer system is a reticle (often called a mask) which includes a pattern thereon corresponding to features to be formed in a layer on the substrate. A reticle typically includes a transparent glass plate covered with a patterned light blocking material such as chrome. The reticle is placed between a radiation source producing radiation of a preselected wavelength (e.g., ultraviolet light) and a focusing lens which may form part of a stepper apparatus. Placed beneath the stepper is the resist-coated silicon wafer. When the radiation from the radiation source is directed onto the reticle, light passes through the glass (in the regions not containing the chrome mask patterns) and projects onto the resist-coated silicon wafer. In this manner, an image of the reticle is transferred to the resist.
The resist (sometimes referred to as the xe2x80x9cphotoresistxe2x80x9d) is provided as a thin layer of radiation-sensitive material that is typically spin-coated over the entire silicon wafer surface. The resist material is classified as either positive or negative depending on how it responds to the light radiation. Positive resist, when exposed to radiation becomes more soluble and is thus more easily removed in a development process. As a result, a developed positive resist contains a resist pattern corresponding to the dark regions on the reticle. Negative resist, in contrast, becomes less soluble when exposed to radiation. Consequently, a developed negative resist contains a pattern corresponding to the transparent regions of the reticle. For simplicity, the following discussion will describe only positive resists, but it is to be understood that negative resists may be substituted therefor.
An exemplary prior art reticle is illustrated in FIG. 1. Prior art FIG. 1 includes a reticle 10 corresponding to a desired integrated circuit pattern 12. For simplicity, the pattern 12 consists of only two design mask patterns. A clear reticle glass 14 allows radiation to project onto a resist-coated silicon wafer. The chrome regions 16 and 18 on the reticle 10 block radiation to generate an image on the wafer corresponding to the desired integrated circuit design features.
As light passes through the reticle 10, it is refracted and scattered by the edges of the chrome 16 and 18. This causes the projected image to exhibit some rounding and other optical distortion. While such effects pose relatively little difficulty in layouts with large features (e.g., features with critical dimensions greater than one micron), they may not be ignored in present day circuit layouts where critical dimensions are about 0.25 micron or smaller. The problem highlighted above becomes even more pronounced in integrated circuit designs having submicron feature sizes near the wavelength of the radiation employed in the photolithographic process.
Prior art FIG. 2 illustrates the impact of the diffraction and scattering caused by the radiation passing through the reticle 10 and onto a section of a photoresist-covered silicon substrate 20. As illustrated, the illumination pattern on the substrate 20 contains an illuminated region 22 and two dark regions 24 and 26 corresponding to the chrome regions 16 and 18 on the reticle 10. The illuminated pattern 22, however, exhibits considerable distortion, with the dark regions 24 and 26 having their corners 28 rounded. Unfortunately, any distorted illumination pattern propagates through the developed resist pattern and negatively impacts the integrated circuit features, such as polysilicon gate regions, vias in dielectrics, etc. As a result, integrated circuit performance is degraded.
To remedy this problem, a reticle correction technique known as optical proximity correction (OPC) has been developed. OPC involves the adding of dark regions to and/or the subtracting of dark regions from portions of a reticle to mitigate the distorting effects of diffraction and scattering. Typically, OPC is performed on a digital representation or simulation of a desired integrated circuit pattern. The digital representation is often referred to as the mask layout data and is used by the reticle manufacturer to generate the reticle. First, the mask layout data is evaluated with software to identify regions where optical distortion will result. Then the OPC is applied to compensate for the distortion. The resulting pattern is ultimately transferred to the reticle glass.
Prior art FIG. 3 illustrates how OPC may be employed to modify the reticle design illustrated in FIG. 1 and thereby provide more accurately the desired illumination pattern at the substrate. As shown, an OPC-corrected reticle 30 includes two features 32 and 34 outlined in chrome on the glass plate 36. Various corrections 38 and 40 have been added to the base features. Some correction takes the form of xe2x80x9cserifs.xe2x80x9d Serifs are typically small, appendage-type addition or subtraction regions typically made at corner regions or other areas on reticle designs.
Prior art FIG. 4 illustrates an illumination pattern 50 produced on a photoresist-covered wafer surface 52 by radiation passing through the reticle 30 of prior art FIG. 3. As shown, the illuminated region includes a light region 54 surrounding a set of dark regions 56 and 58 which substantially faithfully represent the desired pattern illustrated in prior art FIG. 1. Note that the illumination pattern 22 of prior art FIG. 2 which was not produced with a reticle having OPC (reticle 10) has been improved greatly by the reticle 30 having OPC.
Although OPC designs provide performance improvements over features which do not employ OPC as illustrated in prior art FIGS. 1-4, presently it is difficult to determine which OPC design is the optimal design for a given feature, even with the most advanced simulation equipment. As illustrated in prior art FIG. 5, a feature 60 on a mask 62 has a core portion 64 with an OPC design 66 applied thereto. The OPC design 66, however, may include different types of serifs 68a, 68b of various dimensions at various locations about the feature 60. For example, the serif 68a may attach to the core portion 64 at various points and thus may vary substantially in its dimensions. In addition, the serif 68b may have a variable width, a variable length, and may exist at various distances away from the core portion 64. Presently, however, there is not an efficient way of evaluating whether one type of OPC design is better than another in achieving its goal, namely to produce a feature pattern on a substrate which substantially approximates an ideal feature pattern 70, as illustrated in prior art FIG. 6.
Another problem associated with the analysis of OPC designs for a given feature is in analyzing the mask fabrication process which is employed in fabricating the mask. As illustrated in prior art FIG. 7, a portion of mask layout data associated with a core feature 80 having an OPC design 82 is used to generate a pattern on a mask (i.e., a mask pattern). As illustrated in prior art FIG. 7, different mask fabrication processes for a given feature result in mask patterns that approximate the intended feature having the OPC, but nevertheless differ from one another. For example, the mask pattern 86 formed by the mask fabrication process A of prior art FIG. 7 may have been generated using a dry etch while the mask pattern 88 formed by the mask fabrication process B may have been generated using a wet etch which caused the mask patterns 86 and 88 to differ. Given the fact that different mask fabrication processes provide mask patterns which approximate the intended OPC design, but differ from one another, one must evaluate which mask fabrication process is the optimal process to utilize in order to maximize the benefits provided by OPC.
The present invention relates to a system and method of characterizing feature patterns having optically corrected designs by employing an atomic force microscopy (AFM) system for non-linear and non-symmetric critical dimension (CD) measurements. More specifically, the AFM system is configured to provide CD measurements over regions of an optical proximity correction (OPC) corrected feature pattern. The CD measurements from different OPC corrected feature patterns are analyzed to determine which OPC correction provides the closest to the intended design. Based upon the analysis, an optimal OPC design and/or process may be rapidly and efficiently selected. Thus, painstaking manual analysis of critical dimension data as provided by prior art methods is substantially mitigated, and subjective interpretation of the critical dimension measurements is substantially eliminated.
According to one aspect of the present invention, a system and method of characterizing printed feature patterns having different OPC designs is provided. A first OPC design, having a first set of features, forms an input data set and is characterized by spatially measuring critical dimensions of the features. The measured segments from various portions of the features are then combined and presented in a graphical sequence as an image (e.g., a computer monitor display). Another OPC design providing comparable features is also then similarly characterized as a data set and presented as a graphical overlay to the previous image. From the graphical comparison of the two data sets, an efficient and rapid determination may be made as to which design provides the best representation of a desired feature. By graphically observing the displayed features as an overlay, performance characteristics of each OPC design, such as corner rounding, pull-back, and end rounding may be efficiently determined.
Another aspect of the present invention relates to a system for evaluating optical proximity corrected (OPC) designs. The system includes a measurement system for performing measurements relating to a feature pattern having a respective OPC design. The measurement system is configured to characterize at least a portion of the feature pattern as a first image based on the measurements and to employ a second image to facilitate analysis of the first image, whereby a comparison of the first and second images facilitates evaluation of corresponding OPC designs.
Yet another aspect of the present invention relates to a system for optimizing OPC design factors. The system includes a measurement system for providing measurement data from an OPC corrected structure. A processing system operatively coupled to the measurement system is configured to analyze the measurement data from the OPC corrected structure over a predetermined area of the structure. The processing system provides OPC design performance information based upon the measurement data within the predetermined area.
Still another aspect of the present invention relates to a method for optimizing optical proximity correction (OPC) for a mask feature. The method includes measuring corresponding segments of at least two feature patterns having different OPC corrections to provide a set of measurement data for each of the different OPC corrections. The different OPC corrections are characterized based on the respective measurement data set. Performance characteristics for each OPC correction are determined based on an evaluation of each OPC characterization.
Another aspect of the present invention relates to a method for optimizing optical proximity correction (OPC) for a mask feature. The method includes using atomic force microscopy to perform measurements on a feature pattern having a respective OPC design and to provide measurement data according the measurements. The measurement data is evaluated to determine performance characteristics of the respective OPC design, such as, for example, corner rounding, pull-back, and end rounding may be efficiently determined. The measurement data also may be compared with a simulated ideal feature pattern to determine statistically a level of error between the feature pattern and that intended to facilitate selection of an optimized OPC design.